Trillian is the best all-in-one IM application nowadays, both for Windows and OS X (IMHO). Adium was my previous choice but I'd got tired of it's bugs. I don't know it's current situation.
VLC has still no rival in it's area. MPLayer variants are feature poor beneath VLC.
Safari is the fastest browser on OS X but lacks many things Firefox & Chrome have, especially add-ons.
Logic is awesome with Ableton Live. Reason as well.
Lightroom beats Aperture.
All other Adobe applications are already rock solid (Photoshop, After Effects etc.).
Microsoft's Office suit was slow as hell until the 2011 version. iWork is very elegant but the rest of the world still uses MS-Office or Libre Office. Plus it is OS X only (like Logic).
Final Cut Pro was a masterpiece until the hideous X version. Nobody uses it except the golden 9 series.
No need to install Adobe Acrobat Reader as OS X supports PDF files out of the box.
And my last discovery is Clementine of course. It's definitely the best audio player on OS X (and Linux and Windows). It's open source and a fork of the legendary Amarok 1.4 audio player from the KDE Linux world and uses processing resources very low. Plays almost every audio format including FLAC and DOES NOT IMPORT the songs you play into somewhere on the library without asking to the user. Songbird fails the mission becuse it's very processing power and RAM hungry and sluggish. iTunes is only used for my iPad. Nothing else.
By the way, Clementine plays 24bit, 192 Khz audio successfully and IT IS FREE (free on both freedom and free not to pay any bucks).